fix a little error in comments (#15922)
Summary:
There is a little error in the comment, "A->B", so the Task B must start after task A finishes, not "B".
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15922
Differential Revision: D13709579
Pulled By: ezyang
fbshipit-source-id: 735afe83f4532b7c7456da3e96209b3e07071f37
